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Appendix VL


A Three-pin Bridge, Fig. 967, consists of two* cantilevers
/ c and A f, springing from abutments / and ft and the middle
portion is completed by the spans c B and B A resting on pins at c,
B, and A. Sometimes the centre spans consist of drawbridges or
bascules, and in other cases are unlifted; but the construction

has similar advantages in both cases, viz., the bending moment is
almost eliminated, and certainly has a zero value at each pin.
The loads being shewn by vertical lines on the space diagram and
by their values in the force diagram, it is required to draw a link
polygon through the three pins, and the deviation of the links
from the curved line of the bridge will indicate the pending
moment. The loads have been imagined smaller on tbe left than